"Islamic law was far from monolithic, with different schools providing competing accounts. Nevertheless, it was agreed that non-Muslims living under non-Muslim rule could readily be enslaved by Muslims, and their status was heritable, although owners could free as well as bequeath, sell and give slaves. However, although, even among orthodox Muslims, the notion that slaves were properly secured by conquest alone was very far from being observed, non-Muslims living under Muslim rule were protected from enslavement, Christians and Jews being regarded as Peoples of the Book, and thus related to Muslims, and enjoying religious freedom on payment of a poll tax. Thus, for the purposes of ensuring slave labour, Muslim societies were not able to draw on the bulk of the population under their control and had to rely on the slave trade. In India, Islamic rulers, such as the sultans of the Delhi sultanate (1206–1526), used enslavement as a form both of extracting revenues and of punishment, not least for not paying taxes. Fiscal factors were to the fore and territorial expansion was in part financed by the sale of slaves."